Add preliminary support of libtorrent v1.2

[glassez]

With this PR I open a series of PRs that add support of libtorrent v1.2.
Why the series? IMO, it's obvious that a lot of work is often easier to do in parts (if you do not agree with me, you can try to make this job at once).
The main purpose of this PR is making qBittorrent compilable with libtorrent-1.2 that is built with deprecated features enabled so then you can easily proceed with adding more compatibilities doing the rest of jobs in libtorrent-1.2 environment. However, I made some additional corrections along the way.

I'm strongly asking reviewers not to pick on its details too much. Just make sure it doesn't break the current logic (health in the libtorrent-1.1 environment), and let's move on.

[Chocobo1]

In general, it looks good. 👍

[ghost]

Perhaps wait until libtorrent 1.2.1 comes out to merge into 4.2.0?

[sledgehammer999]

Sorry for going offtopic: Are there any pending important PRs that are ready to merge? (so I can take a look the next weekend).

IMO, we should not mix adding compatibility with changing logic and adding new features.

I agree. Big changes are best done in logical increments(if possible). Even if that means more commits. The tradeoff (ie less strain on reviewers), is worth it, IMO.

[glassez]

Are there any pending important PRs that are ready to merge? (so I can take a look the next weekend).

#9591 is important for end-users, IIRC. I think it's time to end it. Even if there are still some shortcomings, it will be easier to fix them later. We have plenty of time before the 4.2 release.

[Chocobo1]

#9591 is important for end-users,

@sledgehammer999
I find it easier to support it if it doesn't come with new custom widgets (from the new files) and I really doubt we really want to maintain them.

[ghost]

@sledgehammer999 I think #10099 is the most important PR right now since it is the most drastic change from 4.1 to 4.2

libtorrent is being actively worked on and I'd bet that by the time qBittorrent 4.2.0 is ready to release, we will have libtorrent 1.2.1 come out.

[Ryrynz]

Yeah that's likely, wouldn't hurt to ask arvidn when it's likely due. It can't be far off as he releases generally within every four months and it's almost been three, so perhaps just days away?

[thalieht]

Guys don't worry about libtorrent version. In qBt 4.2.0, the latest commit from libtorrent RC_1_2 branch will usually be used, just like the RC_1_1 branch was used up until now.

[FranciscoPombal]

@sledgehammer999
Not ready, but important:
#10153
Relevant issue and comments:
#10162 (comment)

[xavier2k6]

@sledgehammer999
64bit cache limit to be lifted.
#10194
typo - BUGFIX: Impove DownloadManager code (glassez) Improve
#10386